1. Field of the Invention
The present invention relates to a nitride semiconductor device including a light-emitting device, such as a laser diode (LD) device or a light-emitting diode (LED), and a light receiving device such as a solar cell, and more particularly to a nitride semiconductor light-emitting device.
2. Description of the Related Art
Nitride semiconductors can have a band gap energy which ranges from 1.95 to 6.0 eV depending on their composition, and hence they have been drawing attention as a material for such semiconductor light-emitting devices as a light-emitting diode (LED) device and a laser diode (LD) device. Recently, as a result of the utilization of these nitride semiconductor materials, a high-brightness blue LED device and green LED device have been put to practical use. These LED devices have a double-heterostructure having a p-n junction and both have an output power exceeding 1 mW.
A conventional LED device basically has a double-heterostructure wherein an active layer comprising InGaN is sandwiched between n-type and p-type clad layers each comprising AlGaN. An n-type contact layer comprising GaN is formed on the n-type clad layer, and a p-type contact layer comprising GaN is formed on the p-type clad layer. This laminate structure is formed on a substrate made of, for example, sapphire.
Basically, an LD device can have a structure similar to that of the above-mentioned LED device. However, most of LD devices have a separate confinement structure in which light and carrier are separately confined. A nitride semiconductor LD device of the separate confinement structure is disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 6-21511. This document shows a light-emitting device of the separate confinement structure in which an InGaN active layer is sandwiched between two light-guiding layers, i.e., n-type GaN and p-type GaN guiding layers. A carrier confinement layer of an n-type AlGaN is formed on the n-type light-guiding layer and another carrier confinement layer of a p-type AlGaN is formed on the p-type light-guiding layer.
Meanwhile, a semiconductor device of a conventional double-heterostructure has an active layer, a first clad layer which is formed in contact with the active layer and which has a larger band gap energy than that of the active layer, and a second clad layer which is formed in contact with the first clad layer and which has a larger band gap energy than that of the first clad layer. This structure is intended for an efficient injection of electrons and holes into the active layer in accordance with energy levels.
Likewise, a conventional nitride semiconductor LD device has an active layer and clad layers disposed thereon which include, for example, a light-guiding layer adjoined by a carrier confinement layer (light confinement layer), each having a progressively increased band gap energy (see, for example, the aforementioned laid-open application).
However, it has been found that a conventional nitride semiconductor device having an indium-containing active layer, and particularly an LD device, of the structure mentioned above, has a low-level light-emitting efficiency. In particular, it has been found that the rise in the device temperature by increasing the current supplied to the device is associated with a serious decrease in the light-emitting efficiency.